1. Field of the Invention
The present invention relates to a passive optical network in which a plurality of subscribers are linked through optical channels and a passive optical coupler to a central office of, for example, a telecommunications company.
2. Description of the Related Art
A passive optical network (PON) typically links n optical network units (ONUs) installed on subscribers' premises to a single optical line termination unit (OLT) at the central office, where n is greater than one. The OLT is linked to the passive optical coupler by a single comparatively long optical fiber. The passive optical coupler is linked to the n ONUs through n comparatively short optical fibers. This type of network topology is attractive because it is considerably less expensive than a network that linked each ONU directly to the central office through a separate optical fiber would be. One description (in Japanese) can be found in the article ‘GE-PON OLT/ONU isshin hikari toranshiba no kaihatsu’ (Development of an optical transceiver for a single-fiber GE-PON OLT/ONU), Tanaka et al., Oki Technical Review, Issue 197, Vol. 71, No. 1, January 2004.
In a PON network, different wavelengths are used for the upstream signals sent from the ONUs to the OLT and the downstream signals sent from the OLT to the ONUs. Signals traveling in the same direction are split or combined at the passive optical coupler. An optical coupler of the fiber type with a bandpass filter function can be employed, as described in the article ‘Faibagata WDM hikari kapura’ (A fiber-type optical coupler) by Iwabata et al., Oki Technical Review, Issue 190, Vol. 69, No. 2, April 2002.
In addition, the signals sent to and received from the ONUs must be multiplexed. Three known methods of multiplexing are time-division multiplexing (TDM), wavelength-division multiplexing (WDM), and code-division multiplexing (CDM).
In TDM, different subscribers' signals or ‘channels’ are placed in different time slots so as not to collide. A description can be found in the article ‘Implementation of a TDM Passive Optical Network for Subscriber Loop Application’ by McGregor et al., J. Lightwave Technology, Vol. 7, No. 11, November 1989. TDM was the first type of multiplexing to be widely used in PON networks, but it suffers from the problem of short reach. Since each channel is compressed into a restricted time slot, its constituent signal pulses become very short. As the optical fiber length between the OLT and ONU increases, increasingly powerful and hence expensive photonic transmitting elements are needed to overcome optical attenuation in the fiber and produce detectable pulse energy at the receiving end. At the present state of the art, expensive photonic transmitting elements are required when the fiber length is more than twenty kilometers (20 km). A TDM PON serving n subscribers requires (n+1) of these expensive photonic elements.
In WDM, a different wavelength is assigned to each subscriber's signals. One study of this system appears in ‘Fault Localization in WDM Passive Optical Network by Reusing Downstream Light Sources’ by Lim et al., IEEE Photonics Technology Letters, Vol. 17, No. 12, December 2005. A problem with WDM is that as the number of subscribers increases, the wavelength channels assigned to different subscribers become closely spaced, and photonic transmitting elements with extremely high wavelength stability are needed to keep the channels separate. This wavelength stability also comes at a high price.
In CDM, the signals transmitted by different subscribers' ONUs are encoded in different ways. Although all subscribers' signals are transmitted at the same time on the same optical wavelength, if the codes are properly designed, different subscribers' signals can be separated at the receiving end. Because the entire time axis is available for carrying each subscriber's upstream and downstream signals, the signals can be generated by comparatively low-power photonic transmitting elements, even for comparatively long-distance transmission. Moreover, the problem of wavelength spacing does not arise, regardless of the number of channels. Descriptions of CDM systems can be found in Japanese Patent Application Publications No. 2001-512919 and No. 2004-282742.
A problem with the use of CDM in PON communication systems is that in order to recover data from the received signals, it is also necessary to recover a clock signal. Since the received signal is a combination of all n subscribers' signals, the received signal has more than n different amplitude (intensity) levels. Clock recovery in this situation is much more difficult than clock recovery from a signal with only two amplitude levels. Japanese Patent Application Publication No. 8-237239 describes a type of device that could be used for clock recovery in a CDM system, but the device is complex and expensive.
The difficulty of clock recovery also complicates the ranging measurement process in which the round-trip time of signals sent to and returned from each ONU is measured.
There is accordingly a need for a PON communication method and system that can serve a large number of subscribers without incurring high equipment costs for photonic transmitting elements or clock recovery devices, even if the OLT-ONU optical distance exceeds 20 km.